It's not worth putting a hardware crypto card in there. At least none that I'm aware of. Card that might actually be effecttive there are not supported in FreeBSD/pfSense.
However it does look like those boxes support 5600 Xeons that do have AES-NI so that might be an option for you. That's based on a brief Google, more research needed!
10 year old hardware though, if it's crashing at all it might be time to replace it.
Edit: Those 5600 Xeons are very cheap now though, probably worth throwing some in there as a test.
Steve